The DnD Drows are charcoal black. The folkloric svartalfr had no real physical descriptions since the definitions between the various creatures are etymological at best. A darker shade of skin IS however an easy short hand and blue, blue grey or purple is not only more easily lit but also looks a bit more fantastic than if they just had a darker earthly skin tone. Nothing wrong with that either of course. But that's the way I figure it.